1. Field of the Invention
The present invention relates to a metal halide lamp substantially not including mercury (Hg), a metal halide lamp apparatus and a vehicle lighting apparatus using the lamp.
2. Description of Related Art
Generally, a metal halide lamp is provided with a discharge vessel filled with an ionizable gas filling including a rare gas, a metal halide, and mercury (Hg). Such a metal halide lamp is practical for use in various light fixtures because of its high efficacy and good color rendering properties.
Particularly, in the view of its high efficacy and a color rendering, it is suitable for such a metal halide lamp to be improved. When the metal halide lamp is used as a vehicle headlight, it must be able to pass a brightness test. The brightness of the lamp shining on a screen must reach a predetermined luminous flux after a predetermined time has elapsed after the vehicle headlight turned on. According to Japan Electrical Lamp Manufactures Association Standard No. 215 (hereinafter JEL-215), a lamp for a vehicle headlight is required to generate its rated luminous flux of 25% one second after the lamp turned on. It is further required to generate its rated luminous flux of 80% four seconds after the lamp turned on.
The mercury (Hg) of a metal halide lamp having mercury (Hg) and a metal halide, primarily emits about four seconds after the lamp is lit. Four seconds later, the metal halide starts to emit, so that the lamp starts to increase its luminous flux. The luminous efficacy of mercury (Hg) is half of that of the metal halide. Therefore, the lamp must be supplied twice as much power as that of an ordinary lamp in order to increase the luminous flux to an acceptable level within four seconds after the lamp turned on. For example, in case of applying the lamp having mercury (Hg) to the vehicle headlight, the lamp lights at a rated luminous flux of 25% in one second, and the lamp can emit the rated luminous flux of 100% in four seconds. However, color characteristics, e.g., a color rendering property or a chromaticity is not good during the initial few seconds after the lamp started. For example, the lamp has an out of white color region on the chromaticity diagram at the beginning of lamp operation. It takes about ten seconds for the lamp""s chromaticity to get into the white color region. Furthermore, for this type of lamp, luminous flux slowly increases at the beginning of lamp operation in comparison with that of a halogen incandescent lamp. If the electrical power is further supplied to the lamp in order to increase luminous flux, it is likely to overshoot the desired steady state level of luminous flux because of increased mercury (Hg) evaporation during the initial second after the lamp turned on. Accordingly, in the view of a initial luminous flux of the lamp, it is difficult for the metal halide lamp having mercury (Hg) to be used as a vehicle headlight.
A metal halide lamp is disclosed in U.S. Pat. No. 4,594,529 (prior art 1). A gas discharge lamp is suitable for using with a reflector as a vehicle headlight. The gas discharge lamp comprises a lamp envelope made of quartz glass having an elongate discharge space. Electrodes are arranged near both sides of the an elongate discharge space. Current-supply conductors, connected to respective electrodes, extend outwardly from vacuum-tight seals.
The lamp envelope is filled with an ionizable gas filling including a rare gas, mercury (Hg), and a metal halide. The lamp envelope has a wall thickness (t) of 1.5 mm to 2.5 mm, and an inner diameter (D) of 1 mm to 3 mm at the midway point between the electrodes. The distance (d) between the tips of the electrodes is 3.5 mm to 6 mm. Each of the electrodes projects a length (l) of 0.5 mm to 1.5 mm into the lamp envelope. The quantity A (mg) of mercury (Hg) used in the lamp is determined as follows: 0.002*(d+4*l)*D2xe2x89xa6Axe2x89xa60.2(d+4*l)*D⅓, wherein the inner diameter (D), the distance (d), and length (l) are expressed in mm. Prior art 1 describes a metal halide lamp, which is horizontally arranged. The lamp operates with high efficiency and contains mercury (Hg) in its bulb. However, mercury (Hg) is harmful to our environment and the amount of mercury used in bulbs should be reduced. Also the arc formed by discharge in the bulb is not vertically spread as desired. Rather, the arc height is contracted. Metal halide lamps not including mercury (Hg) (called a mercury less or a mercury free lamp) are disclosed in Japanese Patent 2,982,198 (prior art 2), Japanese Laid Open Application HEI 6-84,496 (prior art 3), HEI 11-238,488 (prior art 4), or HEI 11-307,048 (prior art 5).
According to the prior art 2, a metal halide lamp is filled with either scandium (Sc) halide or a rare metal halide and a rare gas, and is ignited by a pulse current. The metal halide lamp described in prior art 3 has a metal halide and a rare gas so that its color characteristics do not change even if a dimmer controls the lamp. According to prior art 4, a metal halide lamp can be configured to further include another kind of metal halide (a secondary metal halide), e.g., magnesium (Mg) halide, in addition to its primary metal halide in order to improve its electrical characteristics. The metal halide lamp of prior art 5 includes yet another metal halide (a third metal halide), e.g., indium (In) or yttrium (Y) halide, which has an ionization voltage of 5 to 10 eV and an operational vapor pressure of 1xc3x9710xe2x88x925 atm, in addition to scandium (Sc) halide and sodium (Na) halide. The electrodes of this metal halide lamp do not evaporate too much, so that a discharge vessel does not easily blacken.
In the case of a metal halide lamp not including mercury (Hg), a rare gas primarily slightly illuminates about four seconds after the lamp turned on. The luminous efficacy of the rare gas is lower than that of mercury (Hg). Accordingly, even if the lamp is supplied twice as much power as that of an ordinary lamp in order to increase its luminous flux in four seconds or more, after the lamp turned on, the lamp can not satisfy the aforementioned regulation of JEL-215 sufficiently.
The inventions claimed herein describe metal halide lamps, metal halide lamp apparatus, and vehicle lighting apparatus.
In one embodiment of the invention, a metal halide lamp includes a light-transmitting discharge vessel having a sealed portion, and a pair of electrodes projecting into a discharge space of the vessel. Its (D/L) ratio is in the range of about 0.25 to about 1.5, and a t/L ratio is within about 0.16 to about 1.1, wherein L is an interspace of tips of the electrodes, D is a maximum inner diameter thereof, and t is a maximum wall thickness of the discharge space portion. An ionizable gas filling, which contains a rare gas and a metal halide including at least sodium (Na) or scandium (Sc) and not substantially including mercury (Hg), fills in the discharge vessel. Conductive wires electrically connect to respective electrodes and extend from the discharge vessel.
The inventions also include a metal halide lamp apparatus. A metal halide lamp apparatus includes a metal halide lamp and a ballast. The ballast has a relation between a filling pressure X (atm) of xenon (Xe), and a maximum electrical power AA (W) according to the following formula:
3 less than X less than 15, AAxe2x89xa7xe2x88x922.5X+102.5,
wherein the maximum electrical power AA (W) is a maximum wattage supplied to the lamp in four seconds after the lamp turned on.
The inventions presented herein include a vehicle lighting apparatus. A vehicle lighting apparatus includes a metal halide lamp, a reflector accommodating the metal halide lamp, a front cover arranged to an opening of the reflector, and a ballast.
These and other aspects of the invention are further described in the following drawings and detailed description of the invention.